This application is based on application No. H09-016159 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system for use as a high-resolution image-reading lens in, for example, an image-copying or image-reading apparatus.
2. Description of the Prior Art
Conventionally, image-copying or image-reading apparatuses typically employ a fixed focal length lens that offers a fixed reading magnification. Some lenses employed in such apparatuses are designed to offer variable reading magnification, though the range of magnification they offer is usually very limited. In such cases, magnification is varied either by an electrical means or by an optical means. For example, Japanese Laid-open Patent Applications Nos. H6-94993 and S57-73715 propose using a variable-focus lens as a means to vary magnification optically. On the other hand, increasing attention has recently been paid to the technique of properly correcting chromatic and other aberrations by combining an optical element having an optical power of diffraction with an optical element having an optical power of refraction. For example, Japanese Laid-open Patent Application No. H6-242373 and others propose an objective lens based on this principle for use in an optical disk apparatus.
However, the lenses proposed in the above-mentioned patent applications have the following disadvantages. In the variable-magnification image-reading lens proposed in Japanese Laid-open Patent Application No. H6-94993, the conjugate distance varies greatly, and therefore it is difficult to work out an appropriate mechanical design without making the lens unduly large. In the zoom lens system proposed in Japanese Laid-open Patent Application No. S57-73715, the conjugate distance is fixed, but each constituent lens element is considerably large and the lens system as a whole suffers from too large distortion and other aberrations to be used for high-resolution image-reading purposes. In the objective lens proposed in Japanese Laid-open Patent Application No. H6-242373 for use in an optical disk apparatus, aberrations are corrected only within a narrow band width, and therefore this lens is unfit for use in an optical system that employs a halogen lamp.